Modern imagesetters and platesetters utilize optical scanners to write or record images for subsequent reproduction or to read a prerecorded image at a predefined resolution rate. Such scanners may write or record images on or read prerecorded images from various media including photo or thermal sensitive paper or polymer films, photo or thermal sensitive coatings or erasable imaging materials, an aluminum or other metal base printing plate, or other type media. The medium is typically mounted on an imaging surface which may be planar or curved and then scanned with an optical beam.
The primary components of modern imagesetting and platesetting systems include an image processor to generate and/or edit an image, a raster image processor (RIP) for converting data signals from the image processor into signals which can be understood by a controller which controls the scanning of the optical beam by the imagesetter or platesetter. The imagesetter or platesetter itself typically includes a scan assembly which is often disposed and movable within a drum cylinder in which the recording or recorded medium is mounted. The controller, in accordance with the signals from the RIP and its own programmed instructions, generates signals to control the optical scanning so as to write images on or read images from the medium mounted within the drum cylinder by scanning one or more optical beams over the inside circumference of the drum cylinder while the cylinder itself remains fixed. A typical scan assembly of a cylindrical drum type imager system may include a spin mirror or other optical device to direct the light beam over the inside circumference of the drum cylinder, as will be well understood by one skilled in the art.
Because imagesetting and platesetting systems are often used to record or read different quality images, modern systems are typically designed to allow the recording or reading of images at various imaging resolutions or what is often referred to as different system addressabilities. The imagesetting or platesetting system may be designed to record at any one of multiple resolutions such as 1200 dots per inch (dpi), 1800 dpi, 2400 dpi or 3600 dpi depending on the desired quality of the recording. For example, media recorded with graphics and text for use in printing newspapers may be recorded at a relatively low resolution whereas media recorded with graphics which will subsequently be printed in a high quality photographic publication will typically require a relatively high resolution.
Hence, imagesetting and platesetting systems, as well as other optical imaging systems, often need to be capable of operating at multiple resolutions. In optical scanning devices, such as imagesetters and platesetters, the scanning spot size is typically changed to match, at least approximately, the scan line spacing changes required at different system addressabilities and hence different imaging resolutions. A number of techniques have been proposed to vary the optical beam spot size in imagesetting and platesetting system implementations to allow selectability of different system addressabilities.
For example, it is well known to use different size apertures or apodization of the beam to change the beam spot size to correspond with a selected addressability. Systems utilizing this technique may include an aperture disk having multiple apertures so that the disk can be rotated to place the aperture corresponding to a selected addressability in the path of the optical beam and thereby control the beam spot size impinging upon the medium being imaged to provide the desired resolution. The use of an aperture disk provides a relatively uncomplex and inexpensive means for changing the optical beam spot size on the imaged medium to correspond with a selected system addressability. Furthermore, the use of an aperture disk does not contribute significantly to beam misalignment and/or geometric errors within the system. However, the use of apertures to establish different beam spot sizes can result in a significant reduction in the energy available at the imaging surface whenever the beam spot size impinging on the imaging surface is enlarged.
Continuous focus zoom or telescoping lens arrangements have also been proposed to modify the beam spot size impinging on the imaging surface to correspond with a selected system addressability. Zoom lens arrangements have the advantage of continuous resolution adjustment. Zoom lenses, however, are generally considered to be complex. Zoom lens arrangements typically require that the focus and beam alignment be closely controlled and that, in at least some scanning architectures, geometric errors in the scan line be closely monitored. In practice the complexities and costs of using zoom lenses has generally been considered to outweigh the advantage of continuous adjustment.
Telescoping lenses typically require additional optics for collimating and/or focusing the beam. Typically different sets of additional optical elements will be required dependent upon whether the telescoping lens is in a retracted or extended position, thereby even further increasing the number of optical elements required for a practical implementation. Incorporation of this technique within commercial imaging systems is expensive and requires complex adjustments particularly in high resolution scanning implementations.